[lib] remove asserting if messageStore.threads are equal

Summary:
ENG-3743

This code was live for a long time and recently there were no issues, so I think we can remove it and rely on threads data processed by ops.

Test Plan: N/A, it's hard to test as code I am removing was responsible for testing.

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9682